1. Field of the Invention
The invention relates to a method of forming thin films.
2. Discussion of the Background
Chemical Vapor Deposition (hereinafter CVD) is a process by which a solid film is deposited on a substrate by a chemical reaction of gaseous reactants on or near the surface. CVD is a preferred method for depositing thin films due to its ability to deposit uniformly on relatively complex shapes, and its applicability to a wide range of materials. The types of CVD processes can be categorized according to the nature of the chemical reaction, the type of reaction activation energy, system pressure and temperature and type of reagents. The chemical reactions which may be employed for CVD include decomposition (pyrolysis), reduction, oxidation, hydrolysis, disproportionation, nitride formation, carbide formation, synthesis, and combined reactions. In these processes, the substrate may undergo chemical changes. It can act as a reducing agent, dissolve the deposition component, react exothermally with the deposition compound, or exert a catalytic effect. Selection of a reaction is dependent upon desired thin film composition. The energy required to initiate the reactions listed above can come in different forms. Reactions may be thermally activated, plasma-promoted, photon induced, laser induced, or electron induced.
Besides CVD, sputtering is also widely used for the formation of thin films for chemical microsensors. According to the sputtering process, material is removed from a target, carried by plasma and deposited onto a substrate. This method is considered inefficient since as much as 90% of the energies supplied is transferred into heat.
According to the CVD process, a compound is reduced or dissociated resulting in an adherent coating onto a substrate.
Both these methods suffer drawbacks in that it is difficult to restrict the film deposition to a well-defined, small area of a substrate. The very nature of these deposition methods has required the use of complex masks and etching techniques to restrict the film deposition to a well- o defined small area of a substrate.
In addition to the problems of controlling the area of deposition, neither of these methods allow for the monitoring of the film as it is being deposited. Typically the film formation is controlled by the amount of time that deposition occurs forming a film of a desired thickness. However, no methods presently exist for monitoring the formation of a thin film while it is being formed. Therefore, there is currently no method for selectively depositing a thin film by CVD while directly monitoring the progress of film deposition.